As the fetus develops from a single dependent cell into a fully formed neonate capable of sustained life outside the womb, the physiology of individual organs and the integrated systems of the body undergo substantial developmental changes. Although physiology in the early stages may be crude, and significantly different from that observed in maturity, it usually reflects functional differences that allow the fetus to cope with the challenges of the intrauterine environment, and also with the sudden, extreme changes needed for adaptation to extrauterine life. For example, the presence of fetal haemoglobin in utero allows oxygen to be extracted from the placenta in a very low-oxygen environment compared to after birth. In considering developmental physiology (ontogeny) it is necessary to see fetus, preterm newborn, neonate, infant, child and adolescent as stages of development that merge into each other.

Fetal circulation

The fetus and placenta encompass a unit in which the placenta enables the fetus to eliminate carbon dioxide and metabolic waste products in exchange for oxygen and nutrients from the maternal circulation (Figure FP1). Blood leaves the placenta in the single umbilical vein with an oxygen saturation of approximately 80%. The ductus venosus shunts half of the oxygenated umbilical venous blood through the liver to enter the inferior vena cava (IVC). This mixed IVC blood, with a saturation of 65%, enters the right atrium, but only one-third passes into the right ventricle.